Vehicles used in underground mining operations, either at the cutting face, or for transporting ore, usually move around in an environment of darkness. In any case, they are constrained in their movements by the cramped nature of the tunnels, the severity of the direction changes, for example in the case of a network of perpendicular tunnels, and by the near total darkness prevailing in underground mines. In this visually degraded environment, the risk of collision between a vehicle and any obstacle present in its path, such as the wall of a tunnel, is not insignificant.
As the main source of light is limited to the beams emitted by the head lamps of the miners, locating these vehicles is particularly difficult. Visual detection of these vehicles in an environment of darkness is therefore an important requirement for an operator of an underground mine.
As far as visually detecting an object in an environment of darkness is concerned it is known practice to use a retroreflective device, which is a passive system, intended to highlight the presence of an object by night or in darkness and which is based on the application of the optical system known as a retroreflector.
Retroreflection is the name given to reflection in which the rays of light are reflected in a direction similar to the direction from which they came, this property being maintained despite wide variations in the direction of the incident rays.
A retroreflective device is the name given to a device which, when irradiated directionally, retroreflects a relatively high proportion of the incident radiation.
It is known practice to incorporate retroreflective covering materials into the sidewalls of a tire, particularly a tire for a two-wheeled vehicle. Agreement addendum 87 to UN regulation 88 of 20 Mar. 1958 concerning the adoption of uniform conditions of approval and reciprocal recognition of approval from motor vehicle equipment and parts defines the uniform provisions concerning the approval of retroreflective tires for two-wheeled vehicles. A retroreflective tire, within the meaning of the abovementioned regulation 88, is a tire ready for use comprising a retroreflective circle incorporated into each of the sidewalls of the tire.
Through the materials and shapes used for a retroreflective device, manufacturers are looking for the best light reflection index, limiting losses of intensity through diffusion or absorption into the material. The basic materials are varied and adapted to the various uses. By way of examples and nonexhaustively, the marketplace includes adhesive tapes, technical fabrics, paints, and coating products based on glass microbeads acting as reflective pigment.
It is therefore beneficial to apply a retroreflective covering material to the sidewalls of a tire for a heavy vehicle of the civil engineering type intended to move around in an environment of darkness in order to allow the tire and the vehicle on which it is mounted to be detected visually in an environment of darkness.
A tire comprises a tread intended to come into contact with the ground via a tread surface. The tread surface extends axially, i.e. parallel to the axis of rotation of the tire, between two axial ends. Each axial end of the tread surface is extended radially inwards, namely in a direction perpendicular to the axis of rotation of the tire, by two tread end faces. Each tread end face is extended radially inwards by a sidewall external face. The assembly consisting of a tread end face and the sidewall external face extending it radially inwards constitutes a tire lateral face. What is meant by an external surface of the tire is the assembly made up of the tread surface and the two tire lateral faces, which are symmetric about the equatorial plane of the tire passing through the middle of the tread surface and perpendicular to the axis of rotation of the tire.
In an underground mining environment, the external surface of the tire is generally subjected to mechanical attack, on the one hand because of the presence of numerous somewhat angular blocks of stone on the ground and, on the other hand, because of the aggressiveness of the tunnel walls which are hewn from rock.
This mechanical attack may cause chunks of material to be torn out, particularly from the lateral faces of the tire, or may even cause the tire to puncture. In the event of significant chunking, the tire has to be replaced prematurely in relation to its usual life, leading to additional operating costs. In the event of a puncture, changing the tire may moreover prove difficult given the cramped environment and the vehicle may even become stuck in a tunnel, as a result of a tilting of the axle on which the punctured tire is mounted. In any event, the immobilization of the vehicle results in a loss of productivity.
There is therefore a great benefit, to an operator of an underground mine, in being able visually in an environment of darkness to detect the attacked zones of the tire lateral faces in order to be able to plan in the necessary maintenance operations.